1. Field of the Invention
This invention applies to advanced SiC devices for high-speed, high-power applications such as compact efficient power amplifiers in radar transmitters in airborne and ground-based radar systems and high-power density switching applications such as high-voltage DC-DC converters and inverters.
2. Background of the Technology
Two of the most common types of vertical SiC power transistors are the Static Induction Transistor (SIT) and the Bipolar Junction Transistor (BJT). These devices are described in more detail below.
The SIT is a vertical MESFET or JFET type device wherein the gates are close together resulting in space charge limited current conduction. The device characteristics look much like a triode rather than a conventional FET. The advantages of using an SIT are a result of its high voltage gain and good impedance characteristics, which result in a high power gain. In SiC, the device performance is further enhanced by the high saturation velocity (e.g., 1.5-2xc3x97 that of Si) and high electric field breakdown strength (e.g., 10xc3x97 that of Si). Based on SiC""s high thermal conductivity and suitability for use at high-temperatures, a silicon carbide SIT device should produce substantial improvements over Si technology.
An SIT can have either PN or Schottky gates. Additionally, current in an SIT is controlled by the electric field applied to the drain and gate regions. Most SIT""s in SiC have used Schottky metal gates. See, for example, U.S. Pat. Nos. 5,945,701; 5,903,020; 5,807,773; and 5,612,547. See also Henning et al., xe2x80x9cA Novel Self-Aligned Fabrication Process for Microwave Static Induction Transistors in Silicon Carbide,xe2x80x9d Electron Device Letters, 21, 578-580 (2000). Using a Schottky gate in an SIT or MESFET will typically limit the junction temperature to about 250xc2x0 C. because leakage currents exponentially increase through the Schottky gate with increasing temperature.
Much of the early work on the SIT in SiC focused on developing highly uniform, highly controlled epitaxy layers for the drift and channel regions. The early successes of the device were a direct result of improved epitaxy uniformity through the use of wafer rotation and a better understanding of epitaxial growth mechanisms.
Most of the fabrication difficulties currently being experienced in low-cost volume manufacturing can be traced back to the gate-level processing steps. First, the current-carrying capability of the SIT is highly sensitive to the width of the channel regions, which is set by patterned reactive ion etching (RIE). However, after RIE, it is usually necessary to perform thermal oxidation in order to form a high-quality passivation layer over the device. During this step, the oxidation of the sidewall can occur up to five times faster than the planar Si-face of the SiC surface, resulting in variations in the channel width which can be difficult to control precisely. Further, the oxide must be selectively removed from the gate trench bottom and sidewalls, which usually requires wet chemical etching to ensure sidewall oxide removal. This selective removal of oxide along the sidewall is a very challenging step from both an alignment and process point of view.
Once the oxide is removed from the sidewall, the gate Schottky contact can be formed. Due to the very small geometries involved, the gate metal is typically deposited via sputtering, which can induce damage and lower the Schottky barrier, or by angled evaporation. Achieving this coverage, without forming gate-to-source shorts, is extremely difficult as described by Henning, supra.
An additional performance-related problem with the Schottky gate is that the Schottky Barrier metal is deposited on an etched sidewall of SiC. This etched sidewall is not optimal for Schottky deposition, and results in a lowered barrier. Also, the surface is non-planar resulting in a non-planar space charge region in the channel, which can lead to problems with reproducibility or signal fidelity. The non-planar etched sidewall is a result of slight variations in the photolithography pattern. Although it is possible to demonstrate S-band performance with marginal gain, it is clear that an improved process is needed to ensure lower-cost, more reproducible manufacturing of the SiC SIT.
The SiC BJT has been thought to be an attractive device for microwave applications for quite some time. In fact, simulations by R. J. Trew in 1991 indicated that a 6H-SiC BJT with 0.2 xcexcm thick base doped at 3xc3x971018/cm3 could produce useful power up to 4 GHz in a Class A common-emitter configuration. See Trew et al., xe2x80x9cThe Potential of Diamond and SiC Electronic Devices for Microwave and Millimeter-Wave Power Applicationsxe2x80x9d, Proc of the IEEE, 79, 598-620(1991). Optimizing the base resistance while maintaining the needed gain is important. Thinning the base decreases the base transit time but increases the base resistance. Adding to this difficulty is that in SiC, the Al acceptor level is nearly 200 meV from the valence band edge and thus not fully ionized at room temperature.
Also, the p-type base contact can be difficult to fabricate in SiC because of the large bandgap. In fact, low-resistivity contacts to p-type SiC have only been formed on heavily doped p-type SiC. The reasons for this can be understood from the thermal equilibrium band diagram of the metal-semiconductor interface (q"PHgr"M less than q"PHgr"SiC). In general, the Schottky barrier ("PHgr"B) to majority carrier transport should be reduced as much as possible to provide for an ohmic contact. Since the bandgap and electron affinity ("khgr") of SiC are fixed, the remaining options for reducing the "PHgr"B are to choose a metal with a large work function ("PHgr"M) and also to dope the p-type SiC as heavily as possible. P-type ohmic contacts to SiC often use some variation of Al/Ti alloys, and a contact with specific contact resistance of 1.5 xcexa9xc2x7cm2 on Al-doped samples (NA=2xc3x971019/cm3) has been reported using an Al/Ti alloy. See Crofton et al., xe2x80x9cContact resistance measurements on p-type 6H-SiCxe2x80x9d, Appl. Phys. Lett., 62, 4, 384-386 (1993). The specific contact resistance is a strong function of doping. Although Al melts at approximately 660xc2x0 C., a 90:10 Al/Ti alloy (by weight) is a mixture of solid and liquid phase at temperatures of 950 to 1150xc2x0 C., which are typical anneal temperatures used in the formation of ohmic contacts to SiC. See Crofton, supra. See also N. Lundberg et al., xe2x80x9cThermally stable Low Ohmic Contacts to P-type 6H-SiC using Cobalt Silicidesxe2x80x9d, Solid St. Elect., 39, II, 1559-1565 (1996); and Crofton et al., xe2x80x9cTitanium and Aluminum-Titanium Ohmic Contacts to P-Type SiCxe2x80x9d, Solid St. Elect. (1997).
More recent experiments using a 90:10 Al/Ti alloy have yielded specific contact resistance ranging from 5xc3x9710xe2x88x926 to 3xc3x9710xe2x88x925 on p-type 6H-SiC doped at 1.3xc3x971019/cm. On the same material, pure Ti (with a 1 minute, 800xc2x0 C. anneal) was also used to form ohmic contacts with specific contact resistance ranging from 2-4xc3x9710xe2x88x925 xcexa9xc2x7cm2. Removal of the metals after annealing revealed that the Al-based contact spiked into the SiC, evidenced by pits in the SiC surface up to 2600 xc3x85 deep, while the Ti contact exhibited little interfacial reaction. Thus, although Al-based contacts can yield exceptionally low specific contact resistances, the contact can suffer from poor reproducibility and aluminum oxidation during annealing (Al2O3). See Crofton (1997), supra and Porter et al., xe2x80x9cIssues and Status of Ohmic Contacts to P-type Silicon Carbidexe2x80x9d, Trans. 3d Int""l. High Temp. Elect. Conf. (HiTEC), Session VII, 3-8 (1996). A more complete summary of these problems can also be found in Casady et al., xe2x80x9cProcessing of Silicon Carbide for Devices and Circuits,xe2x80x9d chapter included in xe2x80x9cProcessing of Wide Bandgap Semiconductorsxe2x80x9d, Pearton (ed), William Andrew Publishing and Noyes Publications, 178-249 (2000) (ISBN 0-8151439-5).
One of the most significant improvements possible is the development of a self-aligned base process with heavily doped p+ spacers, while not invoking current crowding, and further development of the p-type ohmic contact. Although there is a need for heavy doping under the base contact, it is difficult to form this layer too close to the active base region without invoking current crowding through the periphery of the base. Conversely, having the contact too far away from the active region increases the effective intrinsic base resistance, which is detrimental to the high-frequency performance. The intrinsic base resistance is already quite high in a microwave BJT, since a thin base is required to produce a short base transit time (tbb), wherein tbb is proportional to the square of the base width. Forming a thin base can require precise epitaxy or implant control and the thin base will naturally have a much higher sheet resistance. Control of the p-type doping in the base with advanced epitaxy growth and processing techniques will be very important to realizing the SiC BJT""s full potential as a high-frequency amplifier. The processing demands will be much higher than in the current power switch development efforts involving SiC BIT""s. See, for example, Ryu et al., xe2x80x9c1800 V, 3.8A Bipolar Junction Transistors in 4H-SiCxe2x80x9d, IEEE Dev. Res. Conf. (2000).
It would therefore be desirable fabricate SiC semiconductor devices such as static induction transistors and bipolar junction transistors using a self-aligned process.
According to a first aspect of the invention, a method of making a silicon carbide bipolar junction transistor is provided. According to this aspect of the invention, the bipolar junction transistor comprises a SiC semiconductor substrate layer of a first conductivity type, a SiC drift layer of the first conductivity type disposed on the substrate layer, a SiC base layer of a second conductivity type different than the first conductivity type disposed on the drift layer, and one or more SiC emitter regions of the first conductivity type disposed on the base layer. The method according to this aspect of the invention comprises: forming a SiC emitter layer on the base layer, wherein the base layer is disposed on the drift layer and wherein the drift layer is disposed on the substrate layer; positioning a mask on the emitter layer; selectively etching the emitter layer through openings in the mask to form raised emitter regions separated by etched regions; and selectively forming SiC base contact regions in the etched regions through the openings in the mask.
According to a second aspect of the invention, a method of making a silicon carbide bipolar junction transistor is provided. According to this aspect of the invention, the silicon carbide bipolar junction transistor comprises a SiC semiconductor substrate layer of a first conductivity type, a SiC drift layer of the first conductivity type disposed on the substrate layer, a SiC base layer of a second conductivity type different than the first conductivity type disposed on the drift layer, and one or more SiC emitter regions of the first conductivity type disposed on the base layer. The method according to this aspect of the invention comprises: positioning a mask on the base layer, wherein the base layer is disposed on the drift layer and wherein the drift layer is disposed on the substrate layer; and selectively depositing SiC of the first conductivity type on the base layer through openings in the mask to form the emitter regions.
According to a third aspect of the invention, a method of making a silicon carbide static induction transistor is provided. According to this aspect of the invention, the static induction transistor comprises a SiC semiconductor substrate layer of a first conductivity type, a SiC drift layer of the first conductivity type disposed on the substrate layer, a plurality of SiC gate regions of a second conductivity type different than the first conductivity type disposed on the drift layer, and a plurality of SiC source regions of the first conductivity type disposed on the drift layer. The method according to this aspect of the invention comprises: forming a source layer of SiC of the first conductivity type on the drift layer, wherein the drift layer is disposed on the substrate layer; positioning a mask on the source layer; selectively etching the source layer thorough openings in the mask to form raised source regions separated by etched regions; and selectively forming the SiC gate regions in the etched regions through the openings in the mask.
According to a fourth aspect of the invention, a method of making a silicon carbide semiconductor device is provided. According to this aspect of the invention, the semiconductor device comprises a SiC semiconductor substrate layer of a first conductivity type, a SiC drift layer of the first conductivity type disposed on the substrate layer, and one or more regions of SiC of a second conductivity type different than the first conductivity type disposed on the drift layer. The method according to this aspect of the invention comprises: positioning a mask on the drift layer, wherein the drift layer is disposed on the substrate layer; selectively etching into the drift layer through openings in the mask to form etched regions; and depositing SiC of the second conductivity type in the etched regions through the openings in the mask to form the SiC regions of the second conductivity type.
According to a fifth aspect of the invention, a method of making a silicon carbide semiconductor device is provided. According to this aspect of the invention, the semiconductor device comprises a SiC semiconductor substrate layer of a first conductivity type, first and second SiC drift layers of the first conductivity type disposed on the substrate layer, and one or more SiC gate regions of a second conductivity type different than the first conductivity type formed between the first and second drift layers. The method according to this aspect of the invention comprises: positioning a mask on the first drift layer, wherein the first drift layer is disposed on the substrate layer; selectively implanting ions into the first drift layer through openings in the mask to form the gate regions; removing the mask; and forming the second drift layer on the implanted surface of the first drift layer to form embedded gate regions. One or more source regions of SiC of the first conductivity type can then be formed on the exposed surface of the second drift layer.